He is currently one of the oldest Roman Catholic bishops and one of oldest Dutch bishops. Ernst had his seminary years in the Diocese of Breda and was ordained a priest on 7 June 1941. Later he taught moral theology at the diocesan seminary although he never went to a university or any school for further theological training.
He is however still considered to be a well-respected intellectual and theologian.
Ernst, at the time vicar-general of the Diocese of Breda, was appointed bishop of the same Diocese on 3 November 1967 after the sudden and untimely death of his predecessor, Gerardus Henricus De Veterinary He was consecrated by Cardinal Bernard Alfrink on 17 December the same year.
He remained bishop of the Diocese of Breda until retirement on 6 May 1992. He stayed on running the Diocese, however, as an Apostolic Administrator for another two and a half years which is a remarkably long period.
Ernst himself consecrated his successor, Martinus Petrus Maria Muskens in 1994, and was in 2007 - still in good shape - also Company-Consecrator when the latter resigned and consecrated the current dignitary Hans van den Hende.
In 1976 the Dutch Catholic Peace Movement,, asked Ernst to be the next President of its organisation, becoming Alfrink"s successor. Ernst, former Professor of Moral Theology at the Diocesan Seminary, had always cherished strong views on for instance nuclear pacifism. Even after his retirement as a Bishop and after the end of his Presidency at he spoke out vehemently against President George West. Bush"s War on Terror.